Recently, there has been a great need for compact and light liquid crystal display devices. A light-weight liquid crystal display device can be provided, for example, by thinning a glass substrate used for a liquid crystal display (e. g., see Japanese Patent Laid-Open Publication No. 2001-013489, Japanese Patent Laid-Open Publication No. HEI-05-061011).
The above inventions have been conceived to make a glass substrate lighter and thinner by mechanically polishing the surface of the glass substrate. A thin glass substrate has degraded strength and is difficult to handle. Therefore, typically, two sheets of glass substrates with a thickness that ensures reliable strength are bonded to each other by a sealant after patterning the transparent electrodes. The terminals are sealed to prevent contamination, for example, by polishing agents. Subsequently, the surfaces of the glass substrates are mechanically polished or etched by, for example, lapping, grinding or blasting. Then, those substrates are cut into cells of specific sizes.
As described above, the thickness of the glass substrates polished is reduced, for example, from 0.7 mm to about 0.4 mm. And for a 14-inch display, the weight of the display could be reduced by about 100 grams. Provided on the reduced glass substrates are switching devices, pixel electrodes, opposing electrodes, or the like, to form a liquid crystal display cell.
However, these mechanical polishing or etching methods reported in the Japanese references above require that a pair of glass substrates bonded to each other be fixed to a dedicated fixture and a two-step polishing operation be performed, i. e., a rough polishing step is performed as a first stage and then a finishing polishing step is performed as a second stage. However, the surface roughness of the glass substrates is increased in the first stage (creation of rough surface) and the light transmittance of the glass substrates is reduced to not greater than 20% from 100%. To restore the light transmittance of the glass substrates, the finishing polishing step is performed on the surfaces of the glass substrates, whose surface roughness is first increased by the rough polishing method. If the finishing polishing step involves the grinding or blasting, a grinding stone is used to polish the surfaces. Thus, flattening the surfaces of the glass substrates (mirror surfaces) leads to restoring the reduced light transmittance of the glass substrates.
However, because the reduced glass substrates have its strength reduced, the glass substrates may break more easily during the finishing polishing step. Accordingly, the thickness of the glass substrates is subject to some restriction to ensure the strength of the substrates. For example, a problem arises when the substrate is reduced to a thickness not greater than 0.3 mm.
Furthermore, the polishing operation causes micro-cracks to appear on the surfaces of the glass substrates, potentially reducing the physical strength. Reduction in the physical strength of the glass substrates becomes more pronounced as the thickness of the glass substrates is reduced. Further, bending, deflection, deformation caused by changes in temperature, and the like have to be taken into account, and therefore, the productivity of liquid crystal display device may suffer because of the reduced thickness.